Electrochemically produced hard chromium layers serve not only the purpose of providing a decorative surface finishing. Rather, hard chromium layers are also applied as functional coatings on conducting and non-conducting workpieces, for example in order to exert a protecting function or to favorably influence the surface properties. Therefore, typical applications are protective hard chromium coatings for reduction of corrosion, wear or friction, as well as structured hard chromium layers on printing rollers facilitating wetting with printing inks, or on punching, embossing and deep-drawing tools so as to optimize manufacturing processes in industry.
EP 0 196 053 A2 and DE 34 02 554 A1 describe methods for electroplating of hard chromium on metallic surfaces from an aqueous electrolyte containing chromic acid, sulfuric acid or sulfate and a sulfonic acid, working with cathodic current yields ≧20%. The composition of the electrolyte is intended to eliminate the risk of detrimental etching of the surface to be coated. However, no structures are generated in the hard chromium layer.
A further electrochemical method for depositing hard chromium layers on workpieces is known from U.S. Pat. No. 5,196,108. The electrolyte used therein contains a molybdenum anion, making it possible to employ a high cathodic current yield. This method does not serve to structure a hard chromium layer.
An electrochemical method of generating structured hard chromium layers is known, for example, from DE 44 32 512 A1. It enables structuring of the hard chromium layer by adding salts, such as salts of the elements selenium or tellurium, to the electrolyte. However, the layers thus generated have a spherical structure, in fact, with spherical shapes having sizes between less than 1 μm and up to several μm. This results in an often non-uniform spherical structure of the hard chromium layer, which structure is not suitable for all applications.